Exhaust gas discharged from diesel engines contains particulates (i.e., particulate matter), and release of such a substance into the atmosphere without any treatment is a main cause of air pollution. Therefore, demand has arisen for strict regulation for such a substance. There has been proposed, as effective means for removing particulate matter, a flow-through oxidation catalyst for combustion of soluble organic fractions (SOFs), or a diesel exhaust gas trapping system employing a diesel particulate filter (DPF) for trapping soot. However, for regeneration of such a DPF, particulate matter trapped therein must be continuously removed through oxidation.
Hitherto proposed continuous regeneration systems include a system employing a catalyst including a carrier made of, for example, an oxide of a metal (e.g., aluminum, zirconium, cerium, titanium, or vanadium) or a composite oxide, and an expensive noble metal (e.g., Pt) supported on the carrier (see, for example, Patent Document 1, 2, or 3); and a continuous regeneration method involving NO2 (see, for example, Patent Document 4). This continuous regeneration method requires provision, upstream of a DPF, of an oxidation catalyst (e.g., Pt) for oxidizing NO into NO2, and thus involves high cost. In addition, reaction involving NO2 is affected by the ratio of NOx to C, and many restrictions are imposed on the employment of this method.    Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. H10-047035    Patent Document 2: Japanese Patent Application Laid-Open (kokai) No. 2003-334443    Patent Document 3: Japanese Patent Application Laid-Open (kokai) No. 2004-058013    Patent Document 4: Japanese Patent No. 3012249